With the development of network technology, the network services grow explosively. And wavelength division multiplexing (WDM) can satisfy the ever-increasing bandwidth demands, so it will be the core technology for future all optical networks undoubtedly.
In WDM based wavelength switch optical networks, the wavelength converters are expensive, so decreasing the use of wavelength converters is necessary. When there are few wavelength converters or no wavelength converter in optical network, to transmit data on end-to-end path, it is necessary to designate a same wavelength at each node of the path, which is known as wavelength continuity constraint (WCC).
The communication network is becoming more huge due to the increase of optical network services. In order to maintain and manage, a big domain is divided into many small domains forming a distributed network. Generally, a domain is a set of network elements within the scope of a common address management or a path computation area of network, such as an Autonomous System (AS) or an Interior Gateway Protocol (IGP) area. For the consideration of confidentiality and security, each domain maintains internal network information itself, such as network topology and available resources, and conceals them from other domains, which brings that the internal network information of other domain is unknown and only few information on topology and resources is exchanged among domains for confidentiality, computing and configuring, i.e. establishing an inter-domain path that satisfies wavelength continuity constraint in multi-domain network is difficult.
Establishing a path in multi-domain optical network, i.e. computing and configuring an inter-domain path that satisfies wavelength continuity constraint in multi-domain network is usually a time-consumption procedure. In a dynamic multi-domain optical network, such long time delay on path establishment will seriously decrease the success of path provision, for resources conflict will arise from that long time delay. For example, the available wavelength detected in path establishment will probably be grabbed by other connections due to the long time delay, and that will ruin the previous efforts. Therefore, to find a method for computing and configuring an inter-domain path efficiently is vital to minimize the influence of wavelength continuity constraint on path establishment.
The path computation element (PCE) architecture proposed by Internet Engineering Task Force (IETF) has high performance in computing inter-domain path. Each PCE maintains a traffic engineering database (TED), which contains the link and resource information of its own domain such as the wavelength utilization information. PCE uses various path computing technologies, such as the backwards recursive path computation (BRPC) to complete the computation of end-to-end inter-domain path.
FIG. 1 is a schematic diagram of inter-domain path establishment based on BRPC in prior art. In which, OXC is optical cross-connected device, TX is the transmitting port of optical signal, and RX is the receiving port of optical signal.
As shown in FIG. 1, the processes of computing inter-domain path based on BRPC are as follows: the source node N11 as path computation client (PCC) sends computation request to PCE1 in its domain. The PCEs needed to complete this computation are predetermined. The PCE1 in the source node domain, i.e. Domain1 will send the computation request to the downstream PCE, i.e. PCE2 in Domain2. When downstream PCE receives the computation request, it will send the computation request to its downstream PCE, i.e. PCE3 in Domain3. When the computation request arrives at the domain which involves the destination node N33, i.e. Domain 3, PCE3 managing Domain3 will stop transmitting the computation request, and compute a virtual shortest path tree (VSPT) which is a multicast tree from all the entry nodes to the destination node in the domain. After the computation, it passes the VSPT to the upstream PCE. The upstream PCE also computes a VSPT from all the entry nodes of its domain to the entry node of the downstream domain, and combine this VSPT with the received VSPT from downstream domain. Then, it passes this combined VSPT to its upstream PCE. With the backwards recursive path computation method, the PCE which manages the domain involving the source node will finally gain a whole VSPT, and it picks out an optimal path from it. Then the optimal path is transmitted to the source node as reply message. Up to now, the inter-domain path computation is finished. It's noted that PCE utilizes a sequential way when computing inter-domain path, this is, when the downstream PCE is computing path, the upstream PCE is waiting, causing a certain delay in time, as shown in FIG. 1. In FIG. 1, (1) (2) represent the submitting of inter-domain path computation and the replying of computation result processes.
When the source node receives the path computation result, it initiates the RSVP-TE protocol to proceed with the second step, the resource configuration, namely, the wavelength reservation at each node, cross connecting the designated wavelength channel in the fiber signal receiving port which connects the upstream node, with the same wavelength channel in the fiber signal transmitting port which connects the downstream node to finish the path establishment.
The resource reservation protocol-traffic engineering (RSVP-TE) is the most popular signaling protocol performing resources reservation or provision. RSVP-TE utilizes the PATH and RESV messages to fulfill resource configuration along the path, including wavelength checking and reservation.
The PATH message including a Label Set (LS) object traverses hop-by-hop from the source to the destination gathering the wavelength information along the path. The LS object is created at the source node containing available wavelength labels on its outgoing downstream fiber link. And it will be updated depending on the actual available wavelengths on links along the path. If one or more wavelengths in the link from current node to the next node are unavailable and these labels also exist in the LS object, these labels will be removed from the LS object. Finally when the PATH message arrives at the destination node, the wavelength labels in the LS object are available wavelength labels of the path. If the LS object is empty, it denotes that no wavelength satisfies WCC along the path at this moment.
When the destination node receives the PAHT message, it chooses one available wavelength label from the LS object, and sends it to the upstream node as RESV message in an opposite direction to actually reserve wavelength on each node. When the RESV message arrives at the source node, the wavelength reservation along the path is successfully completed, and the service data can be transmitted. However, it will take a long time for the node which received the RESV message to reserve wavelength. Wavelength reservation on the node will consume approximately 10-15 ms. Compared with wavelength reservation, message delivery delay is much shorter for its light speed in fiber. One fiber link of 1 Km consumes only 0.0033 ms. So, wavelength reservation will be a long time consuming procedure when the inter-domain path spans more nodes, as shown in FIG. 1 (3), and that will undoubtedly increase the risk of resource conflict.
From the above analysis, it is visible that to inter-domain path establishment, no matter the sequential computation procedure of PCE or the resource reservation, namely wavelength reservation in lower layer, is long time consuming, and wavelength reservation is the main factor of long time delay. Long time delay can increase the risk of resource conflict and blocking probability of establishment of inter-domain path. At the same time, PCE does not engage in resource configuration process of each node or link, so it does not work on wavelength continuity constraint.